1. Field of the Invention
The present invention is related to the field of electro-acoustic transducers. More specifically, the present invention is related to the use of piezoelectric actuators to convert electrical energy to acoustic energy.
2. Description of the Related Art
Electro-acoustic transducers have numerous applications. Among these applications is the use of electro-acoustic transducers in acoustic wireline well logging tools.
Acoustic logging tools are typically used in evaluation of earth formations penetrated by a wellbore. The acoustic logging tool generally is adapted to traverse the wellbore while the tool is being conveyed by a wireline or cable. The tool comprises at least one transducer which converts electrical energy into acoustic energy, and a transducer which converts acoustic energy into an electrical signal which can be processed by circuits in the tool, or transmitted along the cable to processing equipment located at the earth's surface. The transducer which converts the acoustic energy into an electrical signal can either be the same or a different transducer than the transducer which converts electrical energy into acoustic energy. Operation of the tool typically includes the following sequence of events: an electrical energy pulse is sent to the transducer from a circuit in the tool, where it is converted into an acoustic energy pulse; the acoustic energy pulse travels through the wellbore and strikes the earth formation; some of the acoustic energy pulse is returned to the tool by direct reflection and some of the pulse is returned to the tool by internal refraction along the wall of the wellbore; some of the energy returned to the tool is converted by a transducer, which can be the same transducer which emitted the pulse, depending on the type of tool, into an electrical signal; and the electrical signal is processed either by circuits in the tool or by a computer at the earth's surface into information which can be used to determine certain properties of the earth formation.
A common type of transducer operates on the piezoelectric principle. The transducer comprises a material which changes shape upon application of an electric field. Materials having this property are known in the art. Conversely, the application of pressure to the material, which can be applied by acoustic energy, changes the shape of the material, thereby generating a voltage. The voltage generated is precisely proportional to the amount of change in shape for any particular composition of piezoelectric material, which makes piezoelectric transducers desirable for use where precise proportionality in conversion from acoustic energy to electric energy is required.
Piezoelectric transducers are difficult to use because the amount of change in the shape of the transducer is small, even with a large voltage applied to the transducer. It is difficult, therefore, to generate large acoustic signals at low frequencies with a piezoelectric transducer. It is known in the art to combine or "stack" individual layers of the piezoelectric material to increase the voltage response to acoustic energy, but the stacked piezoelectric element may lack sufficient structural strength to acoustically energize the wellbore at very high signal amplitudes without mechanical failure.
It is an object of the present invention to provide a means for increasing the amplitude of an acoustic signal generated by a piezoelectric transducer upon application of electrical energy to the transducer.